11 October 2011

Most methods of phylogenetic analysis deal with simple trees. In these phylogenies, every taxonomic unit has a single direct ancestor (or "parent"). But we know that phylogeny is often more complex than this. Our own species is an excellent example—while we are all primarily descended from one population in Africa, different peoples around the globe have inherited smaller percentages of ancestry from preexisting populations.

A new study by Reich & al. looks in some detail at peoples who have inherited DNA from the Denisovans, a fossil group known from Siberia. Ancient DNA has been retrieved from these fossils, although unfortunately the fossils are otherwise too scant to tell us much about what Denisovans looked like (other than "humanlike").

Reich & al. posit a complex phylogeny wherein populations are often descended from multiple ancestral populations. Lets take a look at the clades posited in this study.

Operational Taxonomic Units

Reich & al. used the following nine populations, seven extant and two extinct, as operational taxonomic units.

Neandertals.—An extinct group of robust near-human peoples from West Eurasia.(Photo by myself, of a sculpture by John Gurche.)

Denisovans.—An extinct group of near-human peoples known from Siberia but thought to have had a wider range.Note: The photo is of a sculpture of Homo heidelbergensis, thought to be the common ancestor of humans, Neandertals, and Denisovans. Denisovans may not have looked exactly like this.(Photo by myself, of a sculpture by John Gurche.)

Phylogeny

Reich & al. postulated the simplest phylogeny that could possibly explain their data. (Note that the actuality is likely more complex than this, but it's a good starting point.) More recent groups are to the right, and the thickness of the lines indicates the percentage of DNA contributed from population to population.

My diagram, not theirs. Any inaccuracies are my own.
Free for reuse under Public Domain.

I've added a line for the Denisovans' mitochondrial (motherline) ancestor, even though it's not part of the paper's phylogeny. More on that as we start looking through the various clades.

For looking at the clades I'll use a different diagram that does not reflect percentage of ancestry, but simply shows direct descent as unweighted arcs connecting parent and child taxonomic units.

Phylogeny of human and near-human populations according to Reich & al. 2011.
Created using Names on Nodes.
Free for reuse under Public Domain.